The 3rd-Generation Partnership Project (3GPP) has developed a third-generation wireless communications known as Long Term Evolution (LTE) technology, as documented in the specifications for the Evolved Universal Terrestrial Radio Access Network (UTRAN). LTE is a mobile broadband wireless communication technology in which transmissions from base stations (referred to as eNodeBs or eNBs in 3GPP documentation) to user terminals (referred to as user equipment, or UEs, in 3GPP documentation) are sent using orthogonal frequency division multiplexing (OFDM). OFDM splits the transmitted signal into multiple parallel sub-carriers in frequency.
More specifically, LTE uses OFDM in the downlink and Discrete Fourier Transform (DFT)-spread OFDM in the uplink. The basic LTE downlink physical resource can be viewed as a time-frequency resource grid. FIG. 1 illustrates a portion of the available spectrum of an exemplary OFDM time-frequency resource grid 50 for LTE. Generally speaking, the time-frequency resource grid 50 is divided into one millisecond subframes. As shown in FIG. 3, each subframe includes a number of OFDM symbols. For a normal cyclic prefix (CP) length, which is suitable for use in situations where multipath dispersion is not expected to be extremely severe, a subframe consists of fourteen OFDM symbols. A subframe has only twelve OFDM symbols if an extended cyclic prefix is used. In the frequency domain, the physical resources are divided into adjacent subcarriers with a spacing of 15 kHz. The number of subcarriers varies according to the allocated system bandwidth. The smallest element of the time-frequency resource grid 50 is a resource element. A resource element consists of one OFDM subcarrier during one OFDM symbol interval.
LTE resource elements are grouped into resource blocks (RBs), each of which in its most common configuration consists of twelve subcarriers and seven OFDM symbols (one slot). Thus, a RB typically consists of 84 REs. The two RBs occupying the same set of twelve subcarriers in a given radio subframe (two slots) are referred to as an RB pair, which includes 168 resource elements if a normal CP is used. Thus, an LTE radio subframe is composed of multiple RB pairs in frequency with the number of RB pairs determining the bandwidth of the signal. In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms. This is shown in FIG. 2.
The signal transmitted by an eNB to one or more UEs may be transmitted from multiple antennas. Likewise, the signal may be received at a UE that has multiple antennas. The radio channel between the eNB distorts the signals transmitted from the multiple antenna ports. To successfully demodulate downlink transmissions, the UE relies on reference symbols (RS) that are transmitted on the downlink. Several of these reference symbols are illustrated in the resource grid 50 shown in FIG. 3. These reference symbols and their position in the time-frequency resource grid are known to the UE and hence can be used to determine channel estimates by measuring the effect of the radio channel on these symbols.
The notion of virtual resource blocks (VRB) and physical resource blocks (PRB) has been introduced in LTE. The actual resource allocation to a UE is made in terms of VRB pairs. There are two types of resource allocations, localized and distributed. In a localized resource allocation, a VRB pair is directly mapped to a PRB pair, hence two consecutive and localized VRB are also placed as consecutive PRBs in the frequency domain. On the other hand, distributed VRBs are not mapped to consecutive PRBs in the frequency domain, thus providing frequency diversity for data channel transmitted using these distributed VRBs.
Messages transmitted over the radio link to users can be broadly classified as control messages or data messages. Control messages are used to facilitate the proper operation of the system as well as proper operation of each UE within the system. Control messages include commands to control functions such as the transmitted power from a UE, signaling to identify RBs within which data is to be received by the UE or transmitted from the UE, and so on.
Specific allocations of time-frequency resources in the LTE signal to system functions are referred to as physical channels. For example, the physical downlink control channel (PDCCH) is a physical channel used to carry scheduling information and power control messages. The physical HARQ indicator channel (PHICH) carries ACK/NACK in response to a previous uplink transmission, and the physical broadcast channel (PBCH) carries system information. The primary and secondary synchronization signals (PSS/SSS) can also be seen as control signals, and have fixed locations and periodicity in time and frequency so that UEs that initially access the network can find them and synchronize. Similarly, the PBCH has a fixed location relative to the primary and secondary synchronization signals (PSS/SSS). The UE can thus receive the system information transmitted in BCH and use that system information to locate and demodulate/decode the PDCCH, which carries control information specific to the UE.
As of Release 10 of the LTE specifications, all control messages to UEs are demodulated using channel estimates derived from the common reference signals (CRS). This allows the control messages to have a cell-wide coverage, to reach all UEs in the cell without the eNB having any particular knowledge about the UEs' positions. Exceptions to this general approach are the PSS and SSS, which are stand-alone signals and do not require reception of CRS before demodulation. The first one to four OFDM symbols of the subframe are reserved to carry such control information. The example shown in FIG. 3 has a control region of three OFDM symbols. The actual number of OFDM symbols reserved to the control region may vary, depending on the configuration of each cell. The particular number n=1,2,3 or 4 for a given cell is known as the Control Format Indicator (CFI), and is indicated by the physical CFI channel (PCHICH), which is transmitted in the first symbol of the control region.
Downlink transmissions in LTE are dynamically scheduled, meaning that in each subframe the base station transmits control information about which terminals data is transmitted to and upon which resource blocks the data is transmitted, for the current downlink subframe. The dynamic scheduling information is communicated to the user equipments (UEs) via the PDCCH, which is transmitted in the control region. After successful decoding of a PDCCH, the UE performs reception of the Physical Downlink Shared Channel (PDSCH) or transmission of the Physical Uplink Shared Channel (PUSCH) according to pre-determined timing specified in the LTE specs. In addition to the PDCCH, the control region in the downlink signal from the base station also contains the Physical HARQ Indication Channels (PHICH), which carry hybrid-ARQ acknowledgements (ACK/NACK) corresponding to uplink transmissions from the UEs served by the base station.
LTE uses hybrid-ARQ (HARQ), where, after receiving downlink data in a subframe, the terminal attempts to decode it and reports to the base station whether the decoding was successful (ACK) or not (NACK) via the Physical Uplink Control CHannel (PUCCH). In case of an unsuccessful decoding attempt, the base station can retransmit the erroneous data. Similarly, the base station can indicate to the UE whether the decoding of the PUSCH was successful (ACK) or not (NACK) via the Physical Hybrid ARQ Indicator CHannel (PHICH).
The downlink Layer 1/Layer 2 (L1/L2) control signaling transmitted in the control region thus consists of the following different physical-channel types:                The Physical Control Format Indicator Channel (PCFICH), informing the terminal about the size of the control region (one, two, or three OFDM symbols). There is one and only one PCFICH on each component carrier or, equivalently, in each cell.        The Physical Downlink Control Channel (PDCCH), used to signal downlink scheduling assignments and uplink scheduling grants. Each PDCCH typically carries signaling for a single terminal, but can also be used to address a group of terminals. Multiple PDCCHs can exist in each cell.        The Physical Hybrid-ARQ Indicator Channel (PHICH), used to signal hybrid-ARQ acknowledgements in response to uplink UL-SCH transmissions. Multiple PHICHs can exist in each cell.        
The PDCCH is used to carry downlink control information (DCI) such as scheduling decisions and power-control commands. More specifically, the DCI includes:                Downlink scheduling assignments, including PDSCH resource indication, transport format, hybrid-ARQ information, and control information related to spatial multiplexing (if applicable). A downlink scheduling assignment also includes a command for power control of the PUCCH used for transmission of hybrid-ARQ acknowledgements in response to downlink scheduling assignments.        Uplink scheduling grants, including PUSCH resource indication, transport format, and hybrid-ARQ-related information. An uplink scheduling grant also includes a command for power control of the PUSCH.        Power-control commands for a set of terminals as a complement to the commands included in the scheduling assignments/grants.        
One PDCCH carries one DCI message with one of the formats above. Since multiple terminals can be scheduled simultaneously, on both downlink and uplink, there must be a possibility to transmit multiple scheduling messages within each subframe. Each scheduling message is transmitted on a separate PDCCH, and consequently there are typically multiple simultaneous PDCCH transmissions within each cell. Furthermore, to support different radio-channel conditions, link adaptation can be used, where the code rate of the PDCCH is selected to match the radio-channel conditions.
Control messages can be categorized into messages that need to be sent only to one UE (UE-specific control) and those that need to be sent to all UEs or some subset of UEs (common control) within the cell being covered by the eNB. Messages of the first type, UE-specific control messages, are typically sent using the PDCCH.
Control messages of PDCCH type are demodulated using CRS and transmitted in multiples of units called control channel elements (CCEs) where each CCE contains 36 REs. A PDCCH message may have an aggregation level (AL) of one, two, four, or eight CCEs. This allows for link adaptation of the control message. Each CCE is mapped to nine resource element groups (REGs) consisting of four RE each. The REGs for a given CCE are distributed over the system bandwidth to provide frequency diversity for a CCE. This is illustrated in FIG. 4. Hence, a PDCCH message can consist of up to eight CCEs, spanning the entire system bandwidth in the first one to four OFDM symbols, depending on the configuration.
Processing of a PDCCH message in an eNB begins with channel coding, scrambling, modulation, and interleaving of the control information. The modulated symbols are then mapped to the resource elements in the control region. As mentioned above, control channel elements (CCE) have been defined, where each CCE maps to 36 resource elements. By choosing the aggregation level, link-adaptation of the PDCCH is obtained. In total there are NCCE CCEs available for all the PDCCH to be transmitted in the subframe; the number NCCE may vary from subframe to subframe, depending on the number of control symbols n and the number of configured PHICH resources.
Since NCCE can vary from subframe to subframe, the receiving terminal must blindly determine the position of the CCEs for a particular PDCCH as well as the number of CCEs used for the PDCCH. With no constraints, this could be a computationally intensive decoding task. Therefore, some restrictions on the number of possible blind decodings a terminal needs to attempt have been introduced, as of Release 8 of the LTE specifications. One constraint is that the CCEs are numbered and CCE aggregation levels of size K can only start on CCE numbers evenly divisible by K. For example, an AL-8 PDCCH message, made up of eight CCEs, can only begin on CCEs numbered 0, 8, 16, and so on.
The LTE Paging Procedure
In LTE networks, a UE is in a RRC_CONNECTED mode or state when a Radio Resource Control (RRC) connection has been established between the UE and the network. Otherwise, the UE is in an RRC_IDLE mode or state. The LTE network uses a paging process to initiate access to a terminal when the UE is in RRC_IDLE mode. Details corresponding to a paging message are scheduled with a DCI message in the common search space, with the Cyclic Redundancy Check (CRC) field of the DCI message scrambled with a P-RNTI. The DCI message points to a corresponding message that is sent on PDSCH. For the purposes of this disclosure, the term “paging message” refers to the control channel message that alerts the UE to the existence of a page. The data carried by the PDSCH and pointed to by the paging message is referred to herein as the “paging message details.”
When the UE is in RRC_IDLE mode, the cell in which the UE is located is generally not known by the network. Therefore the paging message is typically transmitted in each of several cells. These several cells form an entity that is called a tracking area. The tracking area is controlled by the Mobility Management Entity (MME), which keeps track of which tracking area the UE belongs to. The MME is able to do this since the UE reports to the MME whenever it enters a new tracking area.
Paging messages targeted to a given terminal are scheduled for transmission in scheduling occasions that occur in a very sparse manner in time. This approach allows the terminal to be in Discontinuous Receive (DRX) state as much as possible, to save battery power. The subframe in which the terminal wakes up and monitors paging messages is given by a formula that takes into account the identity of the terminal, a cell-specific paging cycle and, optionally, a UE-specific paging cycle.
PDCCH Monitoring
LTE defines so-called search spaces, which define the set of CCEs the terminal is supposed to monitor for scheduling assignments/grants relating to a certain component carrier. A search space is a set of candidate control channels formed by CCEs on a given aggregation level, which the terminal is supposed to attempt to decode. As there are multiple aggregation levels, corresponding to one, two, four, and eight CCEs, a terminal has multiple search spaces. In each subframe, the terminals will attempt to decode all the PDCCHs that can be formed from the CCEs in each of its search spaces. If the Cyclic Redundancy Check (CRC) checks, the content of the control channel is declared as valid for this terminal and the terminal processes the information (scheduling assignment, scheduling grants, etc.). Each terminal in the system therefore has a terminal-specific search space at each aggregation level. These terminal-specific search spaces are collectively called the UE-specific search space (USS).
In several situations, there is a need to address a group of, or all, terminals in the system. To allow all terminals to be addressed at the same time, LTE has defined common search spaces in addition to the terminal-specific search spaces. Again, while there is a common search space for each aggregation level, these are often collectively referred to as the common search space (CSS). The common search space is, as the name implies, common, and all terminals in the cell monitor the CCEs in the common search spaces for control information. Although the motivation for the common search space is primarily transmission of various system messages, it can be used to schedule individual terminals as well. Thus, it can be used to resolve situations where scheduling of one terminal is blocked due to lack of available resources in the terminal-specific search space. More importantly, the common search space is not dependent on UE configuration status. Therefore, the common search space can be used when the network needs to communicate with the UE during UE reconfiguration periods.
A UE thus monitors a common search space and a UE-specific search space in the PDCCH. In each of these search spaces, a limited number of candidates (equivalently, PDCCH transmission hypotheses) are checked, in every downlink subframe for which the UE is in RRC_CONNECTED mode and in a non-DRX interval. For a UE in RRC_IDLE mode, the UE monitors the common search space at least for each paging subframe that is part of the paging cycle. These hypotheses are known as blind decodes, and the UE checks whether any of the transmitted DCI messages is intended for it. The UE knows that the downlink control information is intended for it if the scrambling mask of the CRC of the control message is identical to the expected RNTI of the message. For instance, if a paging message is expected in a subframe, the UE searches the transmitted control channels in that subframe for a message with CRC scrambled with the paging-RNTI (P-RNTI). The UE also monitors other RNTI, such as C-RNTI for scheduling of the shared data channel or the SI-RNTI for scheduling of system information.
The Enhanced PDCCH (ePDCCH)
As of Release 11 of the LTE specifications, UE-specific transmission of control information in the form of enhanced control channels has been introduced. This is done by allowing the transmission of control messages to a UE where the transmissions are placed in the data region of the LTE subframe and are based on UE-specific reference signals. Depending on the type of control message, the enhanced control channels formed in this manner are referred to as the enhanced PDCCH (ePDCCH), enhanced PHICH (ePHICH), and so on.
For the enhanced control channel in Release 11, it has been further agreed to use antenna port pε{107,108,109,110} for demodulation, which correspond with respect to reference symbol positions and set of sequences to antenna ports pε{7,8,9,10}, i.e., the same antenna ports that are used for data transmissions on the Physical Data Shared Channel (PDSCH), using UE-specific RS. This enhancement means that the precoding gains already available for data transmissions can be achieved for the control channels as well. Another benefit is that different physical RB pairs (PRB pairs) for enhanced control channels can be allocated to different cells or to different transmission points within a cell. This can be seen in FIG. 5, which illustrates ten RB pairs, three of which are allocated to three separate ePDCCH regions comprising one PRB pair each. Note that the remaining RB pairs can be used for PDSCH transmissions. The ability to allocate different PRB pairs to different cells or different transmission points facilitates inter-cell or inter-point interference coordination for control channels. This is especially useful for heterogeneous network scenarios.
FIG. 6 shows an ePDCCH that is divided into multiple groups and mapped to an enhanced control region. This represents a “localized” transmission of the ePDCCH, since all of the groups making up the ePDCCH message are grouped together in frequency. In most cases, the groups making up an ePDCCH message are grouped within a single PRB pair, although the largest ePDCCH messages require two PRB pairs.
Note that these multiple groups are similar to the CCEs in the PDCCH. Also note that, as seen in FIG. 6, the enhanced control region does not start at OFDM symbol zero. This is to accommodate the simultaneous transmission of a PDCCH in the subframe. However, there may be carrier types in future LTE releases that do not have a PDCCH at all, in which case the enhanced control region could start from OFDM symbol zero within the subframe.
While the localized transmission of ePDCCH illustrated in FIG. 6 enables UE-specific precoding, which is an advantage over the conventional PDCCH, in some cases it may be useful to be able to transmit an enhanced control channel in a broadcasted, wide area coverage fashion. The frequency diversity provided by this approach is particularly useful if the eNB does not have reliable information to perform precoding towards a certain UE, in which case a wide area coverage transmission may be more robust. Another case where distributed transmission may be useful is when the particular control message is intended for more than one UE, since in this case UE-specific precoding cannot be used. This is the general approach taken for transmission of the common control information using PDCCH.
Accordingly, a distributed transmission over enhanced control regions can be used, instead of the localized transmission shown in FIG. 6. An example of distributed transmission of the ePDCCH is shown in FIG. 7, where the four parts belonging to the same ePDCCH are distributed over the enhanced control regions. 3GPP has agreed that both localized and distributed transmission of an ePDCCH should be supported, these two approaches corresponding generally to FIGS. 6 and 7, respectively. Common control channel transmission using the ePDCCH will be further specified in Release 12 of the 3GPP specifications for LTE.